Method of making dense, conformal, ultra-thin cap layers for nanoporous low-k ild by plasma assisted atomic layer deposition

ABSTRACT

Barrier layers and methods for forming barrier layers on a porous layer are provided. The methods can include chemically adsorbing a plurality of first molecules on a surface of the porous layer in a chamber and forming a first layer of the first molecules on the surface of the porous layer. A plasma can then be used to react a plurality of second molecules with the first layer of first molecules to form a first layer of a barrier layer. The barrier layers can seal the pores of the porous material, function as a diffusion barrier, be conformal, and/or have a negligible impact on the overall ILD k value of the porous material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/673,190 filed on Feb. 9, 2007, which claims priority to U.S.Provisional Patent Application Ser. No. 60/772,572 filed on Feb. 13,2006, the disclosure of which is incorporated herein in its entirety.

DESCRIPTION OF THE INVENTION

1. Field of the Invention

The present invention relates to barrier layers for use in semiconductordevices and methods for their manufacture and, more particularly,relates to barrier layers for porous materials used as interleveldielectrics.

2. Background of the Invention

As device dimensions in semiconductor integrated circuits (ICs) continueto shrink, low dielectric constant (low-k) materials are needed asinterlevel dielectrics (ILD) to mitigate issues caused by reduced linewidth and line-to-line distances such as increasing RC-delay. To satisfythe technical requirements imposed by, for example, the microelectronicsroadmap (where ultra-low k values <2 are specified), future generationILDs will likely incorporate porous materials for use as low-kmaterials. However, the pores of these materials, typically on the orderof angstroms to a few nanometers and connected to each other at elevatedporosities, can trap moisture, gas precursors, and other contaminants insubsequent processes, making practical pore-sealing techniques essentialto ultra low-k implementation.

To be useful for semiconductor integrated circuit applications, apore-sealing coating should be conformal to the 3D topology of patternedILD films. In addition, at the 65 nm or smaller technology node, itshould be less than several nm thick so that its impact on the overallILD k value is negligible. These requirements exclude many thin filmtechniques including, for example, PVD and CVD. One exception is atomiclayer deposition (ALD), for which the coatings are inherently conformaland precisely controlled at sub-nm thicknesses.

Generally, ALD processes form a monolayer of precursor moleculeschemically adsorbed on a surface to be coated. Then, other molecules,for example, in gaseous form, are introduced to react with thatmonolayer so that one atomic layer of the material desired is deposited.Normally there are several layers of molecules adsorbed on the surface.The first layer is a chemically adsorbed layer and has a strong bondwith the surface. The next layers are physically adsorbed layers and areweakly bonded with each other. ALD makes use of this difference betweenchemical adsorption and physical adsorption. At elevated temperatures orreduced partial pressures, over broad ranges, the weakly bondedphysically adsorbed molecules are removed leaving only the saturatedchemisorbed monolayer on the surface. For example, the chamber can bepurged by inert gas or evacuated to a low pressure, to form a saturatedconformal monolayer on the sample surface. Then, the second gas isintroduced to react with the precursor molecules and form an atomiclayer of thin film.

Problems arise using conventional ALD on a porous substrate becauseconventional methods allow molecules to penetrate into the internalporosity of the ILD material, filling pores and drastically increasingthe effective k value. Because ALD is a surface adsorption-baseddeposition process, thin film formation can take place wherever gasprecursor adsorption occurs, including throughout the network ofconnected internal porosity. FIG. 1 depicts a cross section of a portionof a porous material 110 that includes a plurality of pores 120.Conventional ALD forms a barrier layer 130 on the surface of porousmaterial 110, but also forms film 135 within the internal pores therebyincreasing the effective k value of porous material 110. At small ILDfeature dimensions, even short precursor exposure times that reduce theALD penetration depth to, for example, 10 nm, fills a large percentageof the ILD pores.

Thus, there is a need to overcome these and other problems of the priorart to provide barrier layers and methods of forming barrier layers thatare conformal and localized to the surface of a porous material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a barrier layer formed on a porous material byconventional ALD methods in which the internal pores are coated withfilm or portions of film.

FIG. 2 depicts a barrier layer formed in accordance with the presentteachings.

FIGS. 3A-F depict steps in a method for forming a barrier layer inaccordance with the present teachings.

FIG. 4A is cross-sectional transmission electron microscope (TEM) imageshowing a conformal 5 nm thick pore-sealing coating of SiO₂ prepared ona patterned mesoporous low-k silica film in accordance with the presentteachings.

FIG. 48 is an enlarged cross-sectional TEM image showing the interfacebetween a barrier layer and the mesoporous film in accordance with thepresent teachings.

FIG. 5A is a cross-sectional TEM image showing the mesoporous sampletreated by a plasma-assisted ALD (PA-ALD) pore-sealing process and thenexposed to TiO₂ ALD conditions in accordance with the present teachings.

FIG. 5B is a Ti-mapping image in the same area as that of FIG. 5Aacquired with an electron-energy-loss image filtering mode.

FIG. 6 is a cross sectional view of a method for reducing the diameterand/or the chemistry of pores in a porous material in accordance withthe present teachings.

DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings that form a part thereof, and in which is shown by way ofillustration specific exemplary embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention and it is tobe understood that other embodiments may be utilized and that changesmay be made without departing from the scope of the invention. Thefollowing description is, therefore, not to be taken in a limited sense.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Moreover, all ranges disclosed hereinare to be understood to encompass any and all sub-ranges subsumedtherein. For example, a range of “less than 10” can include any and allsub-ranges between (and including) the minimum value of zero and themaximum value of 10, that is, any and all sub-ranges having a minimumvalue of equal to or greater than zero and a maximum value of equal toor less than 10, e.g., 1 to 5.

According to various embodiments of the present teachings depicted inFIGS. 2-5B, methods for forming an ALD barrier layer localized to theimmediate surface of a porous material are provided. In particular, aplasma-assisted ALD (PA-ALD) process is provided that can form a barrierlayer on a porous, low-k material and seal the pores at minimal ILDthickness. As used herein, the term “barrier layer” is usedinterchangeably with the term “cap layer.” A barrier layer formedaccording to the present teachings can seal the pores on a porous ILDmaterial, function as a diffusion barrier, be conformal, localized,and/or have a negligible impact on the overall ILD k value.

Referring to the cross sectional view of FIG. 2, an exemplary barrierlayer 230 is shown. Barrier layer 230 can be disposed on a porous layer210 that can include a plurality of pores 220. In an exemplaryembodiment, porous layer 210 can be a low-k ILD formed of, for example,xero-gel silica or self-assembled surfactant-templated SiO₂. Porouslayer 210 can also be Al₂O₃, a metal, or other porous materials know toone of ordinary skill in the art. In various embodiments, the pores canbe about several angstroms to tens of nanometers in diameter and can bearranged in regular lattice like a crystal. At elevated porosities, thepores 220 can be connected to each other. Porous layer 210 can furtherinclude 3D topology, for example, of patterned ILD layers. Barrier layer230 can be a conformal layer that is confined to the surface of porouslayer 210, seals the interior pores 220, and serves as a diffusionbarrier. Because ALD processes can form one monolayer of the barrierlayer at a time, the total thickness of barrier layer can be controlledso that its impact on the overall ILD k value is negligible.

Turning now to exemplary methods for forming the barrier layers, FIGS.3A-F schematically depict a plasma-assisted ALD process for forming abarrier layer on a porous material according to the present teachings. Aportion of a porous material 310 including pores 320 and a surface 311can be cleaned by methods known to one of ordinary skill in the art,rendering a surface as shown in FIG. 3A. Referring to FIG. 3B, porousmaterial 310 can be placed in a reaction chamber 312 and precursormolecules 340 can be introduced. In various embodiments, precursormolecules can be gas molecules of AX. AX can be, for example, a gaseousor volatile (e.g., a volatile liquid) chemical including two or moreelements or molecules that can provide one or more of the components inthe objective barrier layer material. Precursor molecules 340 can adsorbonto surface 311 of porous material 310. A first layer of precursormolecules 345 can be chemically adsorbed and have a strong bond withsurface 311. Subsequent precursor molecules can be physically adsorbedto first layer of precursor molecules 345, as well as weakly bonded toother precursor molecules 340.

As shown in FIG. 3C, precursor molecules 340 can be removed andsubstantially a first monolayer 345 of precursor molecules chemicallyadsorbed to porous surface 311 can remain. Precursor molecules 340 canbe removed by purging the chamber 312 with an inert gas, such as, forexample, Ar or N₂ or by evacuating the chamber 312. Referring to FIG.3D, a plurality of reactant molecules 350 can be introduced into thechamber 312. Reactant molecules 350, for example, in a gaseous form BYcan be selected to be non-reactive with precursor molecules 345 unlessactivated by a plasma. BY can be, for example, a gaseous or volatile(e.g., a volatile liquid) chemical including two or more elements ormolecules that can react with AX under the influence of a plasma to formAB. One of ordinary skill in the art will understand that reactantmolecules BY and precursor molecules AX are used for illustrationpurposes and that the reactant molecules and precursor molecules can beof other forms. For example, for a SiO₂ barrier layer, the precursormolecules can be HMDS+O2, or TEOS+O2, or other volatile organicSi-precursors that are not pyrophoric in air at room temperatures. For aTiO₂ barrier layer, the precursors can be Ti isopropoxide+O2, or othervolatile organic Ti-precursors that are not pyrophoric in air at roomtemperatures. One of skill in the art will understand that othermolecules for AX and BY that form a barrier layer by plasma assisted-ALDare contemplated.

In plasma-assisted ALD, ions, electrons, and radicals generally movealong straight lines. Once they hit a wall, they will be neutralized andare thus no longer active. As such, plasma does not enter the nanoporesand plasma-assisted ALD does not result in film deposition within thepores. Moreover, plasma-assisted ALD can be operated at room temperaturefurther reducing film deposition within the pores. Plasma with low ionenergies and a plasma source with controllable ion energy can be used tominimize sputtering of chemisorbed layers by ion bombardment.

FIG. 3E shows that under the influence of a plasma 360, an ALD reactioncan occur in which a barrier layer 370 can be formed of AB molecules.Molecules XY 375 can also form. Barrier layer 370 can be formed ofsubstantially a monolayer of AB molecules, span the surface pores, beconformal to surface 311, be confined to the surface 311 of porousmaterial 310, and/or seal the pores of porous material 310. Whileprecursor molecules 340 can be present within the internal pores ofporous material 310, they will not react with reactant molecules 350because the plasma cannot penetrate (and ALD cannot occur) within theinternal porosity. Therefore, no film or portion of film will be formedin the internal pores 320. For purposes of illustration, precursormolecules are not shown within the pores. The chamber 312 can then bepurged with an inert gas or evacuated to remove the XY molecules 375 asshown in FIG. 3F. The steps depicted in FIGS. 3B to 3F can be repeatedto form the desired thickness of barrier layer 370.

Exemplary methods for fabricating the barrier layers are provided belowas Examples 1 and 2 and further explain use of and TEOS and HMDS.

Example 1

An exemplary plasma-assisted process in which ALD is confined to theimmediate surface, allowing pore sealing at minimal ILD thickness isprovided. The purpose of the plasma can be to define the location ofALD. If ALD precursors are chosen to be non-reactive unless activated byplasma, then, ALD can be spatially defined by the supply of plasmairradiation. In this regard one can recognize that the Debye length andthe molecule mean free path in a typical plasma greatly exceed the poredimension of a porous low-k material, thus plasma cannot penetrate (andALD cannot occur) within the internal porosity.

The exemplary method was carried out in a modified plasma-assisted ALD(PA-ALD) system. The deposition chamber was a 25 mm diameter Pyrex tube,evacuated by a turbomolecular pump to a base vacuum of 5×10⁻⁷ Torr. AnRF coil surrounded the Pyrex tube for plasma generation. Samples weremounted in a remote plasma zone for reduced ion bombardment andplasma-heating effects. Oxygen and TEOS (tetraethylorthosilicateSi(OCH₂CH₃)₄) were used as the precursors for SiO₂. In the absence ofplasma, they remain unreactive at room temperature. These precursorswere admitted into the reactor alternately via pneumatic timing valves.A constant Ar flow of 15 sccm was used as the carrier gas as well as thepurging gas.

The mesoporous silica thin film samples were prepared on siliconsubstrates by evaporation-induced self-assembly using Brij-56 as thesurfactant to direct the formation of a cubic mesostructurecharacterized by a continuous 3D network of connected pores withdiameters ˜2 nm. These films exhibited excellent mechanical strength andthermal stability, along with an isotropic k and low surface roughness,which is important for etching or chemical mechanical polishing. At 50volume % porosity, the k value can be 2.5 or less. Prior to PA-ALD, thesamples were patterned by interferometric lithography and etched with aCHF₃/Ar plasma to create 400×400-nm trenches as shown in FIG. 4A. Thenthe photoresist and any residual organics were removed by oxygen-plasmatreatment.

Plasma-assisted ALD was performed by first introducing TEOS vapor intothe reactor, followed by Ar purging to obtain monolayer (orsub-monolayer) adsorption on the sample surface. RF power was thendelivered to the coil, creating an O₂ and Ar plasma to produce activeradicals that convert surface-adsorbed TEOS into reactive silanols andmay promote further conversion to siloxane. After that, the depositionchamber was purged again to remove the residual gaseous products. Theabove steps were repeated 150 times, with each step lasting 5 seconds.

FIGS. 4A and 4B show cross-sectional TEM images of the sample. A 5 nmthick SiO₂ coating is observed as the smooth dark rim bordering thepatterned mesoporous silica feature. The coating was conformal to thepatterned morphology and uniform in thickness. No penetration of theSiO₂ into the porous matrix was observed, and the interface between thecoating and the mesoporous silica film remained sharp.

To verify the pore-sealing effectiveness of PA-ALD, the PA-ALD coatedsample was put into a traditional thermal ALD reactor, where TiO₂ ALDwas performed. It was shown that standard TiO₂ ALD will infiltratesurfactant-templated mesoporous silica, so this experiment was conductedto demonstrate the effectiveness of PA-ALD pore sealing. At 180° C., thePA-ALD coated sample was treated with 100 thermal ALD cycles using TiCl₄and H₂O as the precursors. FIGS. 5A and 5-B show the corresponding TEMimages. FIG. 5A is a regular cross-sectional TEM image, where two ALDlayers were observed. The inner, lighter layer was the PA-ALD SiO₂coating, and the outer, darker layer was the TiO₂ thermal ALD coating.The mesoporous low-k silica appeared completely unaffected, suggestingthat TiCl₄ and H₂O cannot penetrate through the PA-ALD SiO₂ coating toform TiO₂ in the underlying porous silica matrix. This was furthersupported by the Ti-mapping image in FIG. 5B. The bright border in thisimage represented the location of Ti, and corresponded to the TiO₂overlayer shown in FIG. 5A. Comparing the Ti-mapping image (FIG. 5B) tothe original regular TEM image (FIG. 5A), no detectable TiO₂ was foundbeyond the PA-ALD SiO₂ coating. Therefore, the PA-ALD SiO₂ coating,although only 5 nm thick, was pinhole-free and sufficiently dense toseal the pores and protect the underlying porous low-k silica fromexposure to gaseous chemicals.

Concerning the mechanism of room temperature PA-ALD of SiO₂, it is firstnoted that the deposition rate is quite low, 0.03-nm/cycle, compared to0.07-0.08-nm/cycle for conventional NH₃ catalyzed SiO₂ ALD. ConventionalALD uses multiple water/TEOS cycles, where a water exposure serves tohydrolyze ethoxysilane bonds to form silanols, and alkoxide exposureresults in condensation reactions to form siloxane bonds. As for therelated solution-based ‘sol-gel’ reactions, hydrolysis and condensationare bimolecular nucleophilic substitution reactions catalyzed by acid orbase. In PA-ALD, plasma exposure can take the place of hydrolysis,activating the alkoxide surface toward TEOS adsorption. Silanols canform during PA-ALD. However due to the monolayer (or sub-monolayer)≡Si—OH coverage, the extent of surface hydrolysis can be difficult toquantify. Additionally, the plasma can serve a catalytic role bygenerating nucleophilic oxo radicals, ≡Si—OH• that promote siloxane bondformation. At room temperature the extent of these plasma assistedhydrolysis and condensation reactions can be less than for conventionalammonia catalyzed hydrolysis and condensation reactions, explaining thelower deposition rates. Consistent with a low rate of siloxane bondformation is the highly conformal and dense PA-ALD layer indicative of areaction-limited monomer-cluster growth process-confined, as disclosedherein, exclusively to the plasma-activated surface.

Example 2

Higher PA-ALD deposition rates can be obtained by using precursors withstronger surface adsorptions, for example, using HMDS(Hexymethyldislazane, (CH₃)₃SiNHSi(CH₃)₃) compared of TEOS. HMDS hasstronger chemisorption on a sample surface than TEOS due to its morereactive nature to —OH groups on the sample surface as further describedbelow.

In this example, PA-ALD was carried out with the same apparatus asdepicted in Example 1, but the precursors were HMDS and oxygen. Thedeposition procedures were also the same as the procedures in Example 1:first introducing HMDS vapor into the reactor, followed by Ar purging toobtain monolayer (or sub-monolayer) adsorption on the sample surface. RFpower was then delivered to the coil, creating an O₂ and Ar plasma toproduce active radicals that convert surface-adsorbed HMDS into reactivesilanols and may promote further conversion to siloxane. After that, thedeposition chamber was purged again to remove the residual gaseousproducts. Those steps were repeated for 60 cycles. To further enhancethe step of precursor adsorption, the sample stage can be moderatelyheated up to 120° C. In addition, at the end of each PA-ALD cycle, thesample surface can be treated with H₂O vapor to provide more —OH speciesfor surface adsorption in the following cycle. The same cap layer as theone achieved in Example 1 was obtained in Example 2, but the depositionrate in Example 2 was about 0.106 nm/cycle, much faster than using TEOS.

Using HMDS has several advantages, including the following non-limitingexamples: 1) it is easy to obtain monolayer adsorption because of itspassivating —CH₃ final surface, thus a good PA-ALD cap layer can beattained over a broad experiment conditions; 2) HMDS is a common primerused before coating photoresist in semiconductor processing and istherefore a friendly chemical to microelectronics; and 3) HMDS has beenused to cure the damaged low-k (e.g. damaged by intensive plasma duringstripping photoresist). Thus, HMDS can automatically cure the damagedlow-k at the same time when sealing the pores.

As described herein, PA-ALD can seal pores. Additionally with thedemonstrated very high degree of thickness control, it is alsocontemplated that, prior to complete pore sealing, the pore size of themesoporous silica can be progressively reduced in a sub-Å/cycle fashion.This combined with the thin PA-ALD layer thickness can have veryimportant implications for membrane formation, where extremely thininorganic films with precisely controlled pore size could enable thesynthesis of robust mimics of natural ion or water channels of interestfor sensors and water purification.

In another exemplary embodiment, a pore size reducing layer can beformed on a porous material to reduce the size of the pores and/orchange the chemistry of the pores. FIG. 6 is a cross sectional schematicdrawing of a portion of a porous material 610 that includes a pluralityof pores 620 having a diameter of about D₁ or larger. A first porereducing layer 631 can be formed using PA-ALD as disclosed herein. Thefirst pore reducing layer 631 can reduce the pore diameter from D₁ toD₂, where D₁>D₂. In various embodiments, a second pore reducing layer632 can be formed using PA-ALD over first pore reducing layer 631.Second pore reducing layer 632 can further reduce the pore diameter fromD₂ to D₃, where D₂>D₃. According to various embodiments, first porereducing layer 631 and/or second pore reducing layer 632 can furtherchange chemistry in the pores. Further pore reducing layers arecontemplated to reduce the pore size as desired.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A semiconductor device comprising: an interlevel dielectric (ILD)layer comprising a plurality of pores; and a conformal barrier layerdisposed at a surface of the ILD layer and not within the internal poresof the ILD.
 2. The semiconductor device of claim 1, wherein theconformal barrier layer seals a portion of the plurality of poresdisposed at the surface of the ILD.
 3. A method for adjusting a poresize on a porous material comprising: providing a porous material,wherein the porous material comprises a plurality of pores having adiameter equal to or great than a first diameter; chemically adsorbing aplurality of first molecules on a surface of the porous material in achamber; forming a first layer of the first molecules on the surface ofthe porous material; and using a plasma to react a plurality of secondmolecules with the first layer of first molecules to form a first poresize reducing layer on the porous material, wherein the first pore sizereducing layer reduces the diameter of the plurality of pores to lessthan the first diameter.
 4. The method of claim 3, further comprisingforming a second pore size reducing layer on the first pore sizereducing layer to further reduce the diameter of the plurality of pores.5. The method of claim 3, wherein the first layer changes a chemistry inthe pores.